The use of optical fibres for the transmission of data or optical information has increased dramatically in recent years. The heart of such transmission systems is an optical fibre of silica glass or other suitable material which has been clad with an appropriate material to achieve a "light tube" or waveguide along which light energy can travel in a controlled manner. Optical fibres are extremely small (maybe 100 microns in diameter) and when they are incorporated into a data transmission system it is necessarv to effect interconnections between separate lengths of such fibres. The primary function of an optical connector is to provide a low-loss coupling of light energy from one fibre to the next and it is necessary to align, in an extremely precise manner, the cores of the coupled fibres so as to keep the losses at the joint to an absolute minimum.
The best coupling possible between two fibres is achieved by polishing the ends of the fibres to a smooth finish and then directly butting the ends together. Disregarding any fresnel losses at the glass-air interface such a connection should have losses in the order of 0.2 dB. This type of connection requires high precision equipment and is best suited for permanent splices. For repeated connections a more rugged connector is required, but such can lead to increased losses.
There are six main sources of losses in any fibre to fibre coupling system. The greatest losses are due to lateral misalignment, when the mating fibres are not aligned along their central axes. Also, although manufacturers place tight tolerances on the position of the core within the cladding, any eccentricity of the central core is treated as a lateral misalignment condition. Angular losses occur when the central axes of the two fibres are tilted with respect to each other. End separation losses occur when the ends of the mating fibre are separated. Greater separations result in greater losses since light emanating from the end of an optical fibre is projected in the form of a cone. Dirt, surface irregularities and non-perpendicular ends conspire to keep the ends apart and generate losses. Extrinsic connector (intrinsic fibre) losses are caused by variations in the optical parameters of the fibre, including its "numerical aperture" (NA), concentricity of the core, core ellipticity and diameter variations. Finally, fresnel losses occur whenever light passes from one transparent medium into another medium of a different index of refraction, since part of the transmitted light will be lost to a reflected beam. For transmission from glass to air the fresnel losses can be 0.2 dB for each surface. This loss can be eliminated by using index-matching fluids, or reduced by using anti-reflection coatings.
In order to minimize losses such as described above the tolerences of butt-joint connections must be extremely tight. However, any small piece of dirt which enters the joint can drastically increase the losses of the connection and accordingly the ends of the fibre must always be protected from ambient conditions.
The problems associated with connections as described above can be reduced by the use of "expanded beam" technology through which the optical beam diameter is increased from the core diameter of 100 microns up to a more manageable size of a few millimeters. Since the resulting beam is considerably larger than a speck of dirt the losses associated therewith are reduced. Furthermore since one is dealing, relatively speaking with a macro rather than a micro situation all aspects of the connection become simpler, from manufacture, to maintenance.
If a fibre is placed at the focal point of a lens then the beam emerging from the lens is collimated with diameter much larger than that of the fibre core and if each fibre has an appropriate lens the spot image from one will be formed on the other at the focal point of its lens. Expanded beam connectors obviously reduce losses due to lateral misalignment and end separation. However, due to the auto-collimation such connectors increase the losses due to angular misalignment
In principle, if the fibres are positioned at the focal point of the lenses with the same accuracy as with end-to-end butt joint connections the losses should be the same with an expanded beam coupler. Several couplers using expanded beam technology are presently available commercially. One of the easiest lens to use in fibre connectors is the graded index (GRIN) lens.
Cylindrical GRIN lenses are functionally identical to conventional spherical lenses except that they have flat end surfaces. The change in the index of refraction along its axis generates the unique properties of the GRIN lens and lenses can be tailored by the manufacturer to generate a wide range of optical parameters. The length of a lens defines its pitch, or the fraction of a complete wavelength, that is contained within the lens at a particular wavelength. For the production of a collimated beam from a point source it is necessary to use a quarter-pitch lens.
If one quarter-pitch GRIN lens in a joint is tilted by an angle .theta. relative to the other lens then the transmitted image will be displaced relative to the receiving lens axis by an amount given by the equation z=tan .theta./.sub.NoA where .theta. is the tilt angle; No and A are GRIN lens parameters which determine the focal length of the lens, since f=1/.sub.NoA. For different types of specific GRIN lenses the losses due to a tilt angle of 1 degree can range from about 6 dB to well over 10 dB. Furthermore, as the fibre core size decreases the tilt losses will become more severe. In a GRIN lens connector if there is any tilt variation in the lenses or even in the placement of the fibres then the transmitted image will not be focussed on the receiving fibre. It therefore is very desirable to achieve a connector in which the tilt losses are minimized without demanding extremely high (costly) manufacturing tolerances.
The principles stated above apply to other imaging lenses, not just to GRIN lenses. If the image is formed at the focal point of the lens then a tilt through the angle .theta. will produce a translation of z=tan .theta./N.sub.oA .congruent.f tan f .theta. at the fibre end face. For small angles .theta..congruent.tan .theta..